Professor of Physics, University of Cambridge
When it comes to saving the planet, "every little helps!" Or does it?
Maybe if we all do a little, we'll achieve only a little. Newspapers and television programmes are full of suggestions on how we can be more green. But how can we tell what works?
Can we cut fossil fuel consumption enough to save the planet?
As I argue in this week's More or Less on BBC Radio 4, what we need is a single unit of measurement.
I would like to suggest measuring energies in kilowatt-hours, and measuring how fast activities use or produce energy in kilowatt-hours per day.
One kilowatt-hour (1 kWh) is the electrical energy used by leaving a 40 watt bulb on for 24 hours (and it might cost you 10 or 15 pence (15 or 20 US cent)).
The chemical energy in the food we eat to stay alive amounts to about 3 kWh per day. Taking one hot bath uses about 5 kWh of heat. Driving an average car 50 kilometres uses 40 kWh of fuel.
In total, the British lifestyle (and I apologise to international readers but my study is based on British numbers) uses 125 kWh per day per person for transport, heating, manufacturing, and electricity.
So, how do green lifestyle ideas fare under the quantitative spotlight? Well, some of these sacred cows do not fare too well, and some come through with flying colours. Here are four important examples.
Simple individual actions
Here are two simple individual actions. One is useless, one is very effective.
Turning phone chargers off when they are not in use is a feeble gesture, like bailing the Titanic with a teaspoon.
The widespread inclusion of "switching off phone chargers" in lists of "10 things you can do" is a bad thing, because it distracts attention from more effective actions that people could be taking.
In contrast, turning the thermostat down (or the air-conditioning in hot climates) is the single most effective energy-saving technology available to a typical person.
Every degree you turn it down will reduce your heating costs by 10%; and, speaking of Britain at least, heating is likely to be the biggest form of energy consumption in most buildings.
Transport, by numbers
When it comes to transport, people talk about "the hydrogen economy".
But they also talk about electric cars.
The truth is that today's hydrogen-powered vehicles are an energy disaster.
Most prototype hydrogen-powered vehicles use more energy than the fossil-fuel vehicles they replace. The average fossil car in Britain uses 80 kWh per 100 km, the BMW Hydrogen 7 uses 254 kWh per 100 km.
Drivers can recharge their electric cars at power points like this one in London
In contrast, prototype electric vehicles are far more efficient, using ten times less energy: 20 kWh per 100 km or even 6 kWh per 100 km.
Even if the electricity for electric vehicles comes from traditional fossil-fuel power stations, electric cars are still more efficient than petrol cars; and if in the future we switch over the nation's electricity production from fossil fuels to greener sources, then electric vehicles will win by a mile.
Heating, by numbers
Small-scale combined heat and power is another looming mistake.
Yes, combined heat and power (that is, putting individual power stations in each building, generating local electricity and heat to keep the buildings warm) can be a slightly more efficient way of using fossil fuels than the standard way (namely, centralised power stations and local condensing boilers).
But these CHP systems are only about 7% more efficient. And they use natural gas, which is a fossil fuel!
The good news is, there is a much better way to generate local heat: heat pumps.
Heat pumps are back-to-front refrigerators. Powered by electricity, they pump heat into the building from the outside - either from the air, or from the ground.
The best heat pumps, recently developed in Japan, have a coefficient of performance of 4.9; this means that using 1 kWh of electricity, the heat pump delivers 4.9 kWh of heat in the form of hot air or hot water.
This is a far more efficient way to use high-grade energy to make heat, than simply setting fire to high-grade chemicals, which achieves a coefficient of performance of only 0.9.
Decentralized power generation, by numbers
Roof-mounted micro-wind turbines are an utter waste of resources.
They never pay for themselves. A typical "600 W" micro-turbine mounted on a typical roof in England generates only 0.04 kWh per day - that's one twentieth of the power of a lightbulb.
In contrast, roof-mounted solar water heaters are a no-brainer. They really work: even where the sunniness is only about 30%, a modest 3-square-metre panel can supply half of a typical family's hot water: about 3.8 kWh per day, on average.
Installing windmills on your rooftop could be a waste of time and money
So: how can we power a modern lifestyle without fossil fuels?
Individual actions saving 10% here and 40% there will not get us off fossil fuels. To eliminate fossil fuel use, we will surely also need to increase the amount of energy we get from non-fossil-fuel sources.
Even if we imagine strong efficiency measures and smart technology switches, halving our energy consumption from 125 kWh per day per person to 60 kWh per day, we should not kid ourselves about the scale of the energy challenge which would remain.
Britain could, for example, get 60 kWh per day per person by building wind farms with an area equal to Wales (which would deliver on average 20 kWh per day per person) and a hundred more nuclear power stations (which would deliver 40 kWh per day per person).
I am not pro-wind or pro-nuclear: I am just pro-arithmetic.
We need an energy plan that adds up. It is not going to be easy, but it is possible.
BBC Radio 4's More or Less
is broadcast on Friday, 23 April at 1330 BST and repeated on Sunday, 25 April at 2000 GMT.
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